Diet and Diabetes Prevention: Is a Plant-Based Diet the Solution?

Diet plays an important role in the prevention of type 2 diabetes (1,2). There is convincing evidence that intake of foods and dietary components from plant-based sources, e.g., whole grain products, fiber, vegetable fats, and plant protein, is associated with lower risk of type 2 diabetes (3–5). In contrast, higher intake of meat, especially red and processed meat as well as fat from animal sources, has been linked to increased risk of type 2 diabetes (3–6). Bringing both aspects together, some studies investigated whether adherence to plant-based dietary patterns (high intake of plant-based products and low intake of meat and/or animal products) is associated with risk of type 2 diabetes (7–10). A recently published meta-analysis of nine prospective cohort studies (11) showed that greater adherence to a plant-based dietary pattern was associated with a 23% decreased risk of type 2 diabetes compared with a low adherence (Fig. 1). Another systematic review and meta-analysis focused explicitly on vegetarian dietary patterns (excluding meat and meat products) and incidence of type 2 diabetes (12). The findings also indicated a reduced prevalence and incidence of type 2 diabetes for individuals following a vegetarian diet compared with omnivores (Fig. 1). However, the certainty of evidence was evaluated as low (3). A vegan diet is more restrictive, as individuals avoid any animal-based products, including fish, dairy, cheese, and eggs. The evidence for diabetes prevention with a vegan diet is very limited (13) (Fig. 1). However, there is moderate certainty of evidence that in people diagnosed with diabetes, a vegan diet seems to be effective in reducing body weight and HbA_1c values (13). A plant-based diet can be important not only for human health but also for the impact on the environment (14). A sustainable diet where animal-based products, especially red meat and milk/dairy products, are replaced with plant-based products has the potential to reduce greenhouse gas emissions (15). Thus, if plant-based dietary patterns are effective in preventing chronic diseases, including diabetes, and if these dietary approaches have a beneficial environmental impact, then following a plant-based diet achieves two things at once. As summarized above, convincing evidence for single plant-based foods or nutrients exists, showing that higher intakes were related to lower risk of type 2 diabetes. Studies investigating plant-based dietary patterns in relation to incidence of type 2 diabetes are scarce, and the certainty of evidence of these findings is still limited. Therefore, more research is needed on this topic.

In this issue of Diabetes Care, Glenn et al. (16) investigate the prospective association between the Portfolio Diet and incidence of type 2 diabetes in the Women’s Health Initiative (WHI) using data from 145,299 postmenopausal women. The Portfolio Diet was introduced in 2002 as a “dietary portfolio” of cholesterol-lowering foods, including plant sterols, plant protein, nuts, and viscous fibers, focusing mainly on the reduction of LDL cholesterol and, thus, the risk of cardiovascular disease (17). In this study, a prevalidated Portfolio Diet score, including six components (high intakes of plant protein, nuts, viscous fiber, plant sterols, and monounsaturated fatty acids [MUFA] as well as low intake of saturated fats and dietary cholesterol) was applied. After a median follow-up of 16 years, 13,943 incident cases of diabetes were identified. Greater adherence to the Portfolio Diet was associated with a 23% lower relative risk of type 2 diabetes (hazard ratio 0.77 [95% CI 0.72, 0.82]) compared with a low adherence. The association persisted even after adjustment for BMI and was only slightly attenuated. Higher intakes of viscous fibers and plant sterols and lower intakes of saturated fat and dietary cholesterol were the main contributors associated with decreased risk of type 2 diabetes in this study. Intake of MUFAs was extremely low in this population (0–0.24 servings/day for the first to the fifth quintile), raising the question of whether MUFA intake would be associated with incidence of type 2 diabetes in populations with higher intakes. However, in a recently published meta-analysis, no association between MUFA intake and risk of type 2 diabetes was observed (4).

In addition, Glenn et al. (16) analyzed associations between adherence to the Mediterranean diet and Dietary Approaches to Stop Hypertension (DASH) diet (both dietary patterns that are also based predominantly on plant products) and risk of type 2 diabetes. In comparisons with the Portfolio Diet, similar associations were observed (hazard ratio for high vs. low adherence for the Mediterranean diet 0.79 [95% CI 0.74, 0.83] and DASH diet 0.69 [0.64, 0.73]). The Portfolio Diet was more strongly correlated with the Mediterranean diet (r = 0.68) than with the DASH diet (r = 0.54). These findings underline that so far, there is no evidence that one specific dietary plant-based pattern is more beneficial compared with another—quite the opposite: the results indicate that different plant-based or predominantly plant-based dietary approaches can be useful for type 2 diabetes prevention. However, a previously published study has shown that a healthy plant-based diet, characterized by a high intake of fresh and unprocessed products, was associated with a lower risk of type 2 diabetes, while an “unhealthy” plant-based diet was not (7).

The strength of the study by Glenn et al. (16) is the large sample size, long duration of follow-up, and large number of incident cases. However, only postmenopausal women were included in this study, and thus, the findings are not generalizable to those of other groups. Another limitation is that dietary information was assessed through self-report and, thus, measurement error cannot be ruled out. Where diet was assessed more than once, an average score was calculated; however, the authors did not investigate changes over time. In addition, it should be emphasized that the Portfolio Diet was not assigned as an intervention to the women, but the adherence to a Portfolio Diet score was analyzed. For example, the intake of MUFAs (a component of the Portfolio Diet) was extremely low in this cohort, showing that the full spectrum of the Portfolio Diet could not be covered within this cohort. Finally, due to the nature of the observational study design, confounding factors could have influenced the findings. For example, it is likely that individuals who follow a plant-based diet adhere to a healthier lifestyle per se (e.g., nonsmoking, being physical active, being in the normal weight range) or are generally healthier, which could be a possible explanation for this observation. However, the authors adjusted for important confounders and conducted sensitivity analysis by excluding individuals with chronic diseases at baseline, and findings were robust. Nevertheless, residual cofounding cannot be ruled out.

This study by Glenn et al. (16) adds to the evidence that a predominately plant-based diet is associated with lower risk of type 2 diabetes, and the authors showed for the first time that the Portfolio Diet might be an effective approach for diabetes prevention. In the future, randomized controlled trials with an adequate intervention duration on glycemic outcomes (fasting blood glucose, fasting insulin, and HbA_1c) are needed, as no evidence for the Portfolio Diet and these outcomes is available thus far (18). Since we can mainly focus on surrogate markers in randomized controlled trials, prospective cohort studies on type 2 diabetes incidence replicating the findings are also required. Moreover, more studies are warranted for a full understanding of the potential of predominantly plant-based diets, or even diets completely excluding meat or animal products, regarding the prevention of type 2 diabetes.